Metallosalen modified carbon nitride a versatile and reusable catalyst for environmentally friendly aldehyde oxidation

The development of environmentally friendly catalysts for organic transformations is of great importance in the field of green chemistry. Aldehyde oxidation reactions play a crucial role in various industrial processes, including the synthesis of pharmaceuticals, agrochemicals, and fine chemicals. This paper presents the synthesis and evaluation of a new metallosalen carbon nitride catalyst named Co(salen)@g-C3N4. The catalyst was prepared by doping salicylaldehyde onto carbon nitride, and subsequently, incorporating cobalt through Schiff base chemistry. The Co(salen)@g-C3N4 catalyst was characterized using various spectroscopic techniques including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Infrared Spectroscopy (IR), and Thermogravimetric Analysis (TGA). Furthermore, after modification with salicylaldehyde, the carbon nitride component of the catalyst exhibited remarkable yields (74–98%) in oxidizing various aldehyde derivatives (20 examples) to benzoic acid. This oxidation reaction was carried out under mild conditions and resulted in short reaction times (120–300 min). Importantly, the catalyst demonstrated recyclability, as it could be reused for five consecutive runs without any loss of activity. The reusable nature of the catalyst, coupled with its excellent yields in oxidation reactions, makes it a promising and sustainable option for future applications.


Experimental procedure
In a test tube equipped with a magnetic stirring bar and septum was charged with Co(salen)@g-C 3 N 4 (30 mg), aldehydes (1 mmol), and H 2 O 2 (3 mmol).The mixture was heated at 60 °C with stirring until the reaction was complete, followed by cooling to room temperature.Water (10 mL) was added to quench the reaction, and the resulting mixture was then extracted with ethyl acetate (10 mL).The organic layer was dried using MgSO 4 , and the solvent was evaporated under vacuum.In most cases, the reaction products were obtained in high purity and did not require additional purification methods. 1 H and 13 C NMR analysis and comparison of the melting points with literature values confirmed the identity of the compounds.In a few cases, crude products were further purified using silica column chromatography with ethyl acetate and petroleum ether as eluents.

Results and discussion
The characterization of Co(salen)@g-C 3 N 4 crystalline structures was carried out using powder PXRD (Fig. 2).The PXRD patterns 12 of Co(salen)@g-C 3 N 4 showed two major peaks of diffraction at approximately 27.7° and 13.1°, as depicted in Fig. 1.The peak at around 27.7° was identified as the (002) plane, which arises from the interplanar stacking of aromatic systems.A slight decrease in the peak intensity was observed after the incorporation of the Co complex compared to that of unmodified carbon nitride.However, it was observed that the functionalization of g-C 3 N 4 with the Co complex did not alter the crystalline structure as with pure g-C 3 N 4 (Figure S1 in Supporting Information).
The SEM images of Co(salen)@g-C 3 N 4 at different magnitudes were performed to support the structural and spectroscopic features, and get more information about the catalyst, and results are shown in Fig. 3. g-C 3 N 4 was produced through direct calcination of melamine and showed a two-dimensional (2D) structure consisting of stacked thin sheets with wrinkles and irregular shapes.These sheets have noticeable micro-holes on their surfaces, adding to the material's unique characteristics.The 2D structure of g-C 3 N 4 did not change after incorporation of Co(salen) on the surface of carbon nitride (Figure S2 in Supporting Information).
The EDX spectroscopy image of Co(salen)@g-C 3 N 4 is shown in Fig. 4. The EDS spectrum provided in Fig. 4 confirms the presence of the chemical compound CoCl 2 in the g-C 3 N 4 catalyst.The spectrum shows the appearance of elements related to Co (cobalt), Cl (chlorine), O (oxygen), N (nitrogen), and C (carbon).This provides evidence of the presence of these elements and their compounds on the surface of Co(salen)@g-C 3 N 4 catalyst.
The FTIR (Fig. 5) confirms the functionalization of graphitic carbon nitride surface and several characteristic absorption bands appeared in the FTIR spectrum.FTIR spectra of g-C 3 N 4 , showed typical C-N heterocycle stretches at 1251, 1325, 1450, 1578, and 1635 cm −1 , as well as the characteristic breathing mode of triazine units at 889 cm −1 .The broad absorption bands at 3156 cm −1 correspond to the stretching vibrations of hydroxyl bonds associated with absorbed water in the crystal lattice of carbon nitride.These hydroxyl bonds are physically and chemically attached to the crystalline structure of carbon nitride and manifest as a broad peak in the FTIR spectrum.Furthermore, a narrow peak at 619 cm −1 that is due to the intrinsic Co-O stretching vibration indicates the presence of Co in the catalyst.
To assess the thermal stability of the catalyst, thermogravimetric analysis (TGA and TG) was conducted under an air atmosphere, ranging from 50 to 800 °C.During the TGA, the sample's weight change was monitored as it was subjected to increasing temperatures.This analysis provides valuable information about the decomposition and thermal behavior of the carbon nitride polymer, allowing for a better understanding of its stability and potential applications (Fig. 6a,b).According to the observations presented in Fig. 6a, the initial mass loss occurring below 200 °C is primarily attributed to the evaporation of adsorbed water or other volatile impurities present on the surface of the sample.As the temperature increases, the heating process leads to the decomposition of the applied salicylaldehyde and g-C 3 N 4 .This decomposition results in the chemical conversion of the salicylaldehyde into carbon-containing gases and g-C 3 N 4 into nitrogen and carbon-containing gases.The decomposition of salicylaldehyde initiates at 200 °C and is completed by 350 °C.The main region of weight loss occurs between 350 and 550 °C, which corresponds to the decomposition of carbon nitride.At temperatures exceeding 600 °C, a residual weight of 1% is observed, which is attributed to the presence of cobalt oxide content in the catalyst.
After the preparation and characterization of the Co(salen)@g-C 3 N 4 catalyst, its catalytic performance for the oxidation of benzaldehyde and its derivatives was investigated.The model reaction chosen for this study was the oxidation of benzaldehyde using H 2 O 2 .The goal was to optimize the yields and reaction times by varying the amount of catalyst, oxidation conditions, and temperature.The results of these optimization experiments are presented in Table 1.In the control reactions without the catalyst (Table 1, entry 1), the oxidation reaction of benzaldehyde with H 2 O 2 did not proceed readily, resulting in low product yields.However, when metal-containing M(salen)@g-C 3 N 4 catalysts were introduced, the oxidation reaction was promptly initiated.As shown in Table 1, the catalytic activity of the M(salen)@g-C 3 N 4 catalysts followed the trend of Co(salen)@g-C 3 N 4 > Mn(salen)@g-C 3 N 4 > Cu(salen)@g-C 3 N 4 , with decreasing activity observed (Table 1, entries 2-4).The highest yield (98%) of the desired benzoic acid 2a was achieved after 20 h of reaction at room temperature, using 3 equivalents of H 2 O 2 under solvent-free conditions (entry 4).It should be noted that reducing the amount of H 2 O 2 resulted in a decrease in yield (Table 1, entries 5-8).The next step involved investigating the influence of the amount of Co(salen)@g-C 3 N 4 catalyst in the oxidation system.It was observed that increasing the amount of catalyst had a significant impact on the oxidation efficiency.As the amount of catalyst increased, the oxidation yields improved.Specifically, when the catalyst amount was 5 mg, 10 mg, 20 mg, and 40 mg, the oxidation yields were 51%, 68%, 81%, and 98%, respectively (Table 1, entries 9-12).However, the trend of oxidation yields remained stable when the amount of catalyst was further increased to 40 mg (Table 1, entry 12).
The impact of reaction temperature on the rate of oxidation reactions was investigated for the model reaction in the presence of a catalyst.The experiment involved varying the temperature from room temperature to 80 °C and analyzing the resulting product yields.The reaction was carried out under optimized conditions for two hours.The results, as shown in Table 1, indicate that the yield of the product gradually increased as the reaction temperature was raised.At room temperature, the reaction rate was relatively low, suggesting that the reaction was not proceeding efficiently under these conditions.As the temperature was increased to 40 °C, the yield improved to a moderate level, indicating a faster rate of reaction.The highest yield of the product was obtained at 60 °C, where the reaction rate was optimized.This temperature provided the most favorable conditions for the oxidation reaction to occur, resulting in a high yield of the desired product.However, when the temperature was further increased to 80 °C, the yield showed a slight decrease compared to 60 °C, indicating that the reaction might have started to deviate from the optimal conditions (Table 1, entries 13-15).The scope and generality of the oxidation reaction using aldehydes were investigated under the optimized conditions.The results are summarized in Table 2, showing the yields of benzoic acid derivatives obtained from   The scalability of the oxidation method was evaluated by conducting a gram-scale oxidation of benzaldehyde using the optimized conditions.Remarkably, the oxidation of benzaldehyde on a 10 g scale, with a catalyst amount of only 200 mg, resulted in an impressive isolated yield of 98% for the desired benzoic acid product.This gram-scale oxidation demonstrates the practical applicability of the method and its potential for large-scale synthesis.The use of a relatively small amount of catalyst in proportion to the reaction scale further highlights the efficiency and cost-effectiveness of the process.
The reusability of the carbon nitride-based catalyst was investigated in a model reaction with a 5 mmol-scale reaction using 100 mg of catalyst.The results are summarized in Fig. 7, showing the performance of the recovered catalyst in successive runs.In the first and second runs (Fig. 7, entries 1 and 2), the recovered catalyst exhibited excellent reusability, maintaining its efficiency with high product yields.This indicates the stability and robustness of the catalyst, allowing for multiple reaction cycles without significant loss in performance.However, in the third and fourth runs (Fig. 7, entries 3 and 4), a slight decrease in product yield was observed.Overall, the carbon nitride catalyst showed good reusability up to the five-run, with high product yields.
A comparison was made between the prepared Co(salen)@g-C 3 N 4 catalysts for the oxidation of benzaldehyde to benzoic acid with other reported procedures.The comparison highlighted the advantages of the proposed methodology in terms of reaction times, yields, and the environmentally friendly nature of the process.The results in Table 3 demonstrate that the Co(salen)@g-C 3 N 4 catalysts exhibit shorter reaction times, excellent yields, and importantly, offer a greener alternative compared to other methodologies [58][59][60][61][62][63][64] .
Based on the available data and previous reports 64 , a mechanistic proposal for the oxidation of aldehyde to acid catalyzed by cobalt can be outlined (Fig. 8).In this proposed mechanism, cobalt acts as a catalyst to facilitate the reaction.Initially, the cobalt catalyst undergoes coordination and activation of aldehyde and H 2 O 2 .H 2 O 2 acting as a nucleophile, adds to the activated aldehyde with the assistance of the cobalt catalyst, and carbon nitride facelifted the abstraction of hydrogen from H 2 O 2 and the formation of water.It is worth noting that a radical mechanism may also be involved in cobalt-catalyzed reactions, although further investigation is required to elucidate the precise details 64 .

Conclusion
In this study, a novel series of reusable M(salen)@g-C 3 N 4 catalysts (M=Co, Cu, Mn) was synthesized by incorporating metal complexes (salen) onto the g-C 3 N 4 host.These catalysts exhibited exceptional performance for the oxidation of aldehyde derivatives in the presence of H 2 O 2 at mild reaction conditions with short reaction times.Among them, the Co(salen)@g-C 3 N 4 catalyst demonstrated the best catalytic activity and was further optimized for oxidation conditions.The catalysts displayed high efficiency, durability, and recyclability, making them suitable for long-term operations.The method also proved to be robust and reliable, providing a valuable approach for synthesizing benzoic acid derivatives on a larger scale.Moreover, the scalability of the method was demonstrated by achieving high isolated yields on a gram scale, indicating its robustness and reliability for synthesizing larger quantities of benzoic acid derivatives.

Figure 7 .
Figure 7. Reusability of the carbon nitride catalyst in a mole-scale reaction.

Table 1 .
Optimization of reaction parameter on the model reaction.a GC yields b The reaction was repeated for three times.

Table 2 .
Scope and generality of the oxidation reaction using aldehydes.

Table 3 .
Comparison of literature for oxidation of benzaldehyde.